This invention relates to a multi-mode interferometric coupler (MMI coupler), for example for use in a semiconductor amplifier for telecommunications.
The coupler according to the invention may be used for the manufacture of optical components on an InP or AsGa semiconductor (laser, laser modulator, etc.).
One example application is the manufacture of an amplifier outputting an optical power greater than a standard semiconductor amplifier.
Another example application relates to all transmission systems in which a very linear amplifier is necessary.
Multi-mode couplers, and their application to integrated optics, are already known in prior art; examples of couplers and their applications are given in articles by L. B. SOLDANO, Journal of Lightwave Technology, vol. 13, No.4, page 615, 1995 and in the article by P. A. BESSE, Journal of Lightwave Technology, vol. 14, No. 10, page 2290, 1996.
In the field of semiconductor amplifiers, there are standard semiconductor amplifiers and wide area semiconductor amplifiers.
The typical component of a standard semiconductor amplifier is a single mode wave guide on semiconductor, with a core containing a laser type material. When a current is injected, the material introduces a gain and the lightwave is amplified.
FIGS. 1A and 1B show the variation of the total power and the maximum power respectively, in the same section of this type of standard semiconductor amplifier. In the example given, a light power of xe2x88x9225 dBm is injected, and the total output power is 0 dBm. The maximum power varies in the same way.
Wide area amplifiers can increase the output power of the device by ensuring that the maximum power density does not reach the saturation power level. This saturation power is fixed only by the material and the current. The wave guide is gradually widened to achieve this. Although the wave guide becomes multi-mode, the lightwave remains coupled with the main mode and gradually widens.
The result is that the gain remains the same (25 dB) but the saturation power increases by about 7 dB. FIGS. 2A and 2B show the variation of the total power and the maximum power respectively, in the same section of a wide area semiconductor amplifier.
This type of device has two disadvantages:
(i) it is difficult to couple output light in a single mode wave guide or in an optical fiber,
(ii) the structure is potentially unstable with respect to a local power modification inducing a variation in the index, which induces coupling of the wave in a higher mode, and another local power modification, etc.
Finally, the paper by K. HAMAMOTO published in EICO""97, on Apr. 2-4, 1997, Stockholm, describes an MMI in which all the active material in the coupler is an amplifier.
Compared with these known devices, the multi-mode interferometric coupler according to the invention comprises two parts, one amplifying part and one part made of a transparent material, which guides the radiation amplified in the first part.
The structure according to the invention can be used to make an amplifier with approximately the same gain and the same saturation power as a wide area amplifier. It also enables coupling of all amplified light in a single mode guide, with minimum losses. Finally, the multi-mode interferometric coupler according to the invention does not have the instability characteristic of a wide area amplifier since, due to its multi-mode nature, the invention is not very sensitive to a fluctuation in the index.
Compared with the device described in the article by K. Hamamoto mentioned above, only part of the multi-mode coupler is used as an amplifier. In the first part of the MMI according to the invention, the optical power is deconcentrated and therefore it is advantageous to amplify the radiation in it. In the second part of the MMI according to the invention, the optical power is concentrated, for example on an output guide, and it is important that it should not be amplified to avoid saturating the amplifier. Therefore, the Hamamoto device does not take advantage of selective amplification in areas in which the optical power is low, unlike the device according to this invention.
Furthermore, the device described by Hamamoto does not use any part made of transparent material, but is simply an amplification device.
A single mode guide may be placed at the output from the coupler according to the invention.
Furthermore, the amplifying material may be a structure embedded in an InP substrate.
The amplifying material may be a laser material, for example a quaternary InGaAsP alloy. This material may also have quantic wells.
The invention also relates to an optical amplifier comprising an optical preamplifier and a coupler according to the invention as described above.
The invention also relates to a number of processes:
to amplify the power of a light source,
or to compensate for the losses of an optical fiber,
or to amplify signals multiplexed in wave length, these various processes making use of a coupler or an optical amplifier according to the invention.